The invention relates generally to print heads for ink jet printers. The invention relates more particularly to an ink jet printer having an ink jet print head which can be displaced in the direction of a line of print between side walls of a printer frame, and having an ink filling device for filling the ink jet print head.
Ink jet print heads have a limited ink supply, which is generally sufficient for printing about 1-4 million printed characters. In the case of ink jet printers which must be available day and night, on weekends and even during holiday periods, such as those in fax machines, for example, and in uses with a high permanent loading, such as in cash register printers, the ink supply stored in the ink jet print head is not sufficient.
It has therefore already been proposed, in Japanese Patent Document JP-A-90 29 991, to refill an ink jet print head while it is installed in an ink jet printer. For this purpose, the ink jet print head is equipped with level detectors, which detect its minimum and maximum permissible ink level. If the level falls below the minimum, the ink jet print head is transported with its feed opening underneath an ink refilling device where, with the aid of an electromagnet, a nozzle connected to an ink reservoir which is placed higher up is lowered into the feed opening and is lifted off the latter after the maximum level has been reached.
This arrangement ensures that the ink jet print head is always ready to print. On the other hand, the amount of ink transferred into the ink jet print head is determined by electrically controlled means and depends only on a level detector. If the voltage supply fails during a filling operation, or if the level detector fails, the ink flow from the ink reservoir cannot be interrupted.
U.S. Pat. No. 4,178,595 discloses an ink jet printer in which the ink jet print head is arranged such that it can be displaced in the direction of a line of print between side walls of the printer frame. The feed opening is arranged on one side wall of the ink jet print head, and a discharge opening of an ink filling device is arranged on one side wall of the printer frame, aligned coaxially with the feed opening. In this case, the feed opening can be connected to the discharge opening in a liquid-tight manner by displacement of the ink jet print head in the direction of a line of print toward the discharge opening.
The discharge opening of the ink filling device is fitted to a variable-volume cylinder which, when its volume is increased, removes ink from an ink reservoir and, when its volume is reduced, discharges ink through the discharge opening. The cylinder is dimensioned such that it picks up and discharges only a small amount of ink in each case. When its volume is reduced, a connecting line between the cylinder and the ink reservoir is closed, so that no ink can flow from the ink reservoir into the cylinder or back from the cylinder into the ink reservoir. The reduction in volume of the cylinder is carried out by a piston which can be displaced with the aid of an electromagnet, the increase in volume is carried out by a spring which pushes the piston back. The amount of ink to be transferred into the ink jet print head is determined by the period during which current is applied to the electromagnet.
The known ink filling device additionally needs an electromagnet and additional outlay on control for the electromagnetic. In addition, the electromagnet has to be of powerful design, since it has to produce the flow pressure for the ink, and has to overcome the friction of the piston in the cylinder and the restoring force of the spring.